Recently, research and development of high density optical disk system capable of recording and/or reproducing information by the use of a semiconductor laser emitting violet light with wavelength of about 400 nm are rapidly progressed. For example, by an optical disk system for recording and/or reproducing information according to the specification of NA0.65 and light source wavelength of 407 nm, can reduced information of 20 to 30 GB per on one surface of a 12 cm optical disk which is the same size as a DVD having NA of 0.65, light wavelength of 650 nm, memory capacity of 4.7 GB (cf. patent document 1).
Patent document 1: Japanese Patent Tokkai No. 2003-303436.